ESR and ENDOR investigations of phenanthrenesemiquinones

Abstract

A series of phenanthrenesemiquinone–diphenylthallium complexes were prepared in solution from the corresponding quinones. ESR, ENDOR and TRIPLE spectra provided the proton coupling constants and the unambiguous assignment to distinct molecular positions. The thallium splitting becomes considerably less negative if the semiquinones are twisted by steric hindrance in the 4- and 5-positions. On the other hand, substitution in the 1- and 8-positions renders aTI more negative. These results are interpreted in terms of steric hindrance of the ion-pair solvation.